Relative motion between contacting components in a machine (e.g., high performance machines or engines) can result in excessive wear of one or both components. For example, components subjected to high frequency and low frequency vibrations may result in excessive wear of component attachments or mating surfaces. The component wear, if left undetected, can cause component and machine malfunctions. For example, spring clips in combustion turbine engine experience surface wear from contact with other components due to operational vibrations and dynamic forces.
In some applications, component wear can be controlled to acceptable levels by using lubricants, by employing materials with high resistance to wear and/or by design features that limit motion and contact and resulting component wear. However, there are many situations where relative motion cannot be eliminated, such as in brake linings, meshing gears, contacting sliders and slip fits; wear is unavoidable in such applications.
Knowledge of the wear condition of critical components can be used to avoid forced outages due to unexpected component failures. Such knowledge also enables the machine to be shut down for repair of the worn components at a convenient scheduled time, rather than continuing operation until a component is worn beyond repair or an emergency shut down is required. Significant costs can be saved by both avoiding forced outages and by ensuring the worn parts can be repaired instead of scrapped when a scheduled outage is performed.
The extent of wear and the suitability of the component for continued service can be determined by visual and/or dimensional inspection. In some applications, wear indicators are embedded within or proximate one or more of the contacting surfaces. For example, in the context of brake linings, wear limit notches or “squealers” generate an audible warning when a predetermined amount of lining wear has occurred.
However, there are many applications where periodic inspection is not feasible due to such factors as, for example, time and labor expenses, cost of inspection and operational disruptions due to inspection down time. In addition, visual and audible warnings are not always feasible monitoring solutions, as is the case when monitoring internal components of a gas turbine engine. Thus, there is a need for a system that can monitor component wear while the component is in an operational state.
Wear sensors mounted in one or both of the wearing components can advantageously provide real-time monitoring of component wear during machine operation. These sensors measure the amount of wear that occurs in regions prone to wear and notify an operator when a preselected amount of wear has occurred. The sensors improve machine reliability and enable more accurate maintenance planning. Such monitoring also improves safety and reduces operating and maintenance costs by indicating a maintenance requirement before any component damage occurs. Real time wear monitoring also avoids unscheduled outages.
A conductive wear sensor is described in commonly-owned U.S. Pat. No. 7,270,890, entitled, Wear Monitoring System with Embedded Conductors. The patent describes a sensor comprising a closed circuit conductive trace that is transformed to an open circuit condition when a counterface wears through the conductive trace. While this sensor has many applications, frequently both members of a wear couple (i.e., two components in contact along the wear surface) are electrically conductive metals. An open circuit cannot be detected in such an electrically conductive component.